Sexual reproduction involves the fusion of male and female gametes (sperm and egg) forming a zygote. Asexual reproduction does not involve gametes and can occur through mitosis, fission, budding, or parthenogenesis. Most animals reproduce sexually but some can reproduce asexually. Sexual reproduction has internal cycles and is controlled by hormones in response to environmental cues like seasons. Fertilization can be internal or external. Reproductive organs like ovaries, uterus, testes produce gametes and support gestation and birth.

1. Animal Reproduction

2. Asexual vs. Sexual Reproduction
⚫ In sexual reproduction, the fusion of haploid
gametes forms a diploid cell, the zygote. The
animal that develops from a zygote can in turn
give rise to gametes by meiosis. The female
gamete, the egg, is a large, nonmotile cell. The
male gamete, the sperm, is generally a much
smaller, motile cell.
⚫ Asexual reproduction is the generation of new
individuals without the fusion of egg and sperm.
In most asexual animals, reproduction relies
entirely on mitotic cell division.

3. ⚫ Many invertebrates can reproduce asexually
by fission, the separation of a parent
organism into two individuals of
approximately equal size.
⚫ Also common among invertebrates is
budding, in which new individuals arise from
outgrowths of existing ones.
◦ Ex. In certain species of coral and hydra, new
individuals grow out from the parents body.
⚫ Parthenogenesis is a form of asexual
reproduction in which an egg develops
without being fertilized.
Asexual Reproduction

4. Asexual Reproduction

5. ⚫ The vast majority of eukaryotic species
reproduce sexually.
⚫ Most animals exhibit cycles in reproductive
activity, often related to changing seasons. In
this way, animals conserve
resources, reproducing only when sufficient
energy sources are available and when
environmental conditions favor the survival of
offspring.
◦ For example, ewes (female sheep) have a reproductive
cycle lasting 15-17 days.
⚫ Ovulation is the release of mature eggs and
occurs at the midpoint of each cycle.
⚫ Reproductive cycles are controlled by
hormones, which in turn are regulated by
environmental cues.
◦ Common environmental cues are changes in day
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fall, and lunar cycles.

6. Sexual Reproduction

7. ⚫Hermaphrodism is when each
individual has both male and female
reproductive systems. Because each
hermaphrodite produces as both male
and female, any two individuals can
mate.
◦ This is present in stationary animals, such as
barnacles, burrowing animals, such as clams,
and some parasites, such as tapeworms.
Sexual Reproduction Continued

8. ⚫Fertilization is the union of sperm and
egg, which can be either external or
internal.
⚫External fertilization – the female
releases eggs into the
environment, where the male fertilizes
them
⚫Internal fertilization – sperm are
deposited in or near the female
reproductive tract, and fertilization occurs
within the tract
External vs. Internal Fertilization

9. Internal
External

10. ⚫In many insect species, the female
reproductive system includes a
spermatheca, a sac in which sperm may
be stored for extended periods, a year or
more in some species.
⚫In many nonmammalian vertebrates, the
digestive, excretory, and reproductive
systems have a common opening to the
outside, the cloaca.
Reproductive Organs

11. ⚫Ovaries – the female gonads are a pair of
ovaries that border the uterus and are
held in place in the abdominal cavity by
ligaments (ovaries produce eggs and
reproductive hormones)
⚫Uterus – where eggs are fertilized and
development of the young occurs
Female Reproductive Organs

12. Female Reproductive Organs

13. ⚫Testes – consists of many highly coiled
tubes surrounded by several layers of
connective tissue, this is where the sperm
is formed
Male Reproductive Organs

14. Male Reproductive Organs

15. Spermatogenesis –
the formation and
development of
sperm, is
continuous and
prolific in adult
males
Spermatogenesis

16. Oogenesis – the
development of
mature eggs, is a
prolonged process
in the human
female
Oogenesis

17. ⚫ Human gestation is divided into three
trimesters of about three months each.
◦ First, Second, and Third Trimester
⚫ Placenta – a structure in the pregnant uterus
for nourishing a viviparous fetus with
mother’s blood supply; formed from the
uterine lining and embryonic membranes
⚫ Labor – the process by which childbirth
occurs; a series of strong, rhythmic uterine
contractions
⚫ Of course, protection is always an option ;)
Conception and Birth